This invention relates to an automatic printing apparatus and, more particularly, to an apparatus for adapting a screen printing machine for the printing of cylindrical or tapered objects.
Screen printing is widely used for printing objects of revolutions such as bottles, cans, cones, and the like. Various types of screen printers are available to accomplish printing of three-dimensional objects, and many, such as that taught in U.S. Pat. No. 3,090,300 utilize an object holder or nest mounted on a shaft extending outwardly from the printer. The screen can be held stationary and the squeegee moved relative to both the screen and the object holder to force the ink through the screen for printing flat objects, or, the squeegee may be held stationary against the screen, and the screen and object may moved, as when printing objects of circular dimensions. For objects of revolution, e.g. circular items, one of these elements is moved in timed relationship to one another during the printing cycle, e.g., the object to be printed ordinarily being rotated while the screen is moved laterally, so that the screen and peripheral surface of the object move at identical linear speeds.
In the past, the timed relationship of the movement of the screen, object, and squeegee has been accomplished by a direct drive mechanism, utilizing a rack and pinion gear system. Such a mechanism is taught in each of U.S. Pat. Nos. 3,090,300 to Dubuit; 3,139,824 to Derrickson; 4,111,118 to Green et al.; and 4,184,427 to Bubley et al. In general, the drive carriage of prior art printing presses is reciprocated on a stationary shaft by a suitable drive means, such as a Geneva cam arrangement or the like. On one side of the drive carriage is a rack which cooperates with a pinion gear for rotating an output shaft extending underneath the screen and connected at an outer portion to a nesting apparatus for holding the object to be printed. The output shaft is mounted on the frame so that upon lateral movement of the drive carriage the output shaft will rotate, thereby rotating the object to be printed.
With such a printing accessory, as noted in U.S. Pat. No. 3,139,824 to Derrickson, the size of the pinion gear is crucial to obtaining a correct speed of revolution of the object for accurate printing. For example, for each object to be printed which is of a different diameter, a different pinion gear of a size directly relating to the diameter or radius of curvature of the object to be printed must be substituted so that the gear ratio relative to that of the rack provides movement of the screen at the same linear rate as the periphery of the bottle being rotated. The casting and machining of individual gears can be expensive, and, in order for a screen printer to have the capability of screen printing a wide variety of bottle and pail sizes and the like on short notice, an equally large variety of such expensive gears must be on hand. Should a printer wish to screen print on an object of nonstandard dimensions, this would necessitate the custom machining of a new gear for each such application, causing undue delay and time and an unnecessary large expenditure of money.